It is known that one may preserve the dynamic range of an audio signal transmitted through a path or medium of relatively limited dynamic range by compressing the signal prior to transmission and then expanding the signal subsequent to transmission. This transmission technique is known as "companding" (a contraction of compression-expansion). Examples of syllabic audio signal companders are given in U.S. Pat. No. 3,732,371 which issued May 8, 1973, to R. S. Burwen and U.S. Pat. No. 4,220,429 which issued Sept. 2, 1980, to Talbot et al.
It is also known that one may improve the "apparent" dynamic range of audio signals by means of an expander even though the signals may be of uncompressed form. Examples of audio signal expanders for such "play-back only" applications are given in the article "High Fidelity Volume Expander" by N. C. Pickering in the September, 1947, issue of AUDIO ENGINEERING magazine and in U.S. Pat. No. 3,980,964 which issued Sept. 14, 1976, to R. M. Grodinsky.
A problem common to audio signal expanders (for either compressed or uncompressed signals) concerns avoidance of undesirable psycho-acoustical effects commonly referred to as "pumping" or "breathing". These effects occur in some systems when a change in signal level occurs which results in a change in the expander gain that is not masked or "covered up" by the program material. This problem is particularly troublesome when the amplitude of the audio input signal applied to the expander suddenly changes as occurs, for example, during a musical crescendo or decrescendo. In order to properly expand such transient signal with minimal audible "side effects", it is customary to employ control circuits in the expander which quickly respond to transient signal level changes but which respond more slowly for dynamically slowly changing signals. For this reason, expanders generally employ some form of adaptive or non-linear filtering to vary the expansion characteristic under differing dynamic conditions of the signal to be expanded.
As an example, in the arrangement proposed by Talbot et al., the input signal to be expanded is applied to a detector which produces a control voltage related to the input signal envelope. The control voltage is applied via a parallel combination of a diode and a first resistor to the control terminal of a gain control amplifier in the main audio signal path. The control terminal is also coupled to ground via a series connection of a second resistor and a capacitor.
In operation, for relatively small input signal level changes, the diode is not forward biased and the resistors and capacitor provide a relatively long time constant for smoothing the control signal. As a result, the rate of change of gain of the amplifier is limited to a value which minimizes audibility of gain changes. However, for large input signal transients, the diode becomes forward biased so that the control voltage is coupled essentially instantaneously to the amplifier gain control terminal thereby enabling immediate expansion of the input signal by the amplifier. The capacitor, which normally smooths the control voltage, is prevented from being excessively charged for brief transients because of the presence of the second resistor. Consequently, the gain of the amplifier quickly returns to its previous value subsequent to a brief transient signal condition.
Other examples of noise reduction systems in which a semiconductor diode is employed as a threshold device for modifying the time constant of an adaptive control signal filter are given, for example, in U.S. patent application Ser. No. 229,743 of Christopher et al., filed Jan. 29, 1981, and U.S patent application Ser. No. 258,432 filed Apr. 28, 1981, now U.S. Pat. No. 4,398,157, of C. B. Dieterich, a continuation in part of application Ser. No. 229,518 filed Jan. 29, 1981, now abandoned, the applications being assigned to the common assignee of the present invention. See also, the article by J. Roberts entitled "$70 Decoder for New CX Records" which was published in January, 1982, issue of the magazine POPULAR ELECTRONICS, pp. 39-44.
In the interest of cost reduction and improved reliability, it would be desirable to construct audio expanders, of the general kind discussed above, in integrated circuit form. It would be further desirable to operate the integrated circuit at a relatively low supply voltage level so as to obtain benefits such as reduced power consumption, reduced heat build-up and improved reliability.
Generally speaking, adaptive filter circuits which are designed to operate at a given supply voltage when using conventional silicon diodes as threshold conduction devices may be "scaled down", so to speak, to operate at a lesser supply voltage by substituting diodes having proportionally lower threshold voltages (e.g., germanium diodes, Shottky barrier silicon diodes, etc.) for the silicon diodes. Such a substitution, however, presents certain practical difficulties with regard to integrated circuit fabrication.
As an example, direct substitution of diodes of different materials or construction on the integrated circuit "chip" may require a further processing step (e.g., an added metalization or a deposition of a further semiconductor material). This may reduce the process yield. An alternative of connecting the "substitute" diode to the integrated circuit via external pin connections avoids the need for extra processing steps but requires additional circuit pins which may not be available in the desired integrated circuit package. Also, additional assembly costs may be involved in connecting the substitute diode to the extra pins and reliability may be degraded by the added connections. A further difficulty is that the "substitute" diode may have forward or reverse bias conduction characteristics which are greatly different from the desired "scaled down" values.